Wide range speed control systems have been developed for use in a number of disparate applications. An example of such a system is the Band-Scan FPL Control System by Sequential Electronic Systems, Inc. ("Jitterless Photorecon Systems", Electromechanical Design, December 1965). Another example can be seen in the article by Lamar Cowger, "The Solution to Extremely Slow Motion Control," Drivers & Controls Int'l., Jan. 1982. Further, the system of BEI of Little Rock, Ark., U.S. Pat. No. 3,974,428 issued Aug. 10, 1976 to Hafle, discloses a speed control system which uses an optical encoder to generate the position signals of a motor shaft, or moveable member. The encoder is constructed and arranged to produce a phase variable output signal at the modulation frequency; the phase of the output signal being compared to a reference signal. Based on the phase and frequency of the above two signals, the speed of the drive motor for the rotary member is increased or decreased.
Both of the aforementioned systems are expensive and, in the case of the BEI system, use optical encoders whose signal outputs must be carefully shaped to produce low distortion sine and cosine waveforms. The ATAR tape recorder by the Datatape Division of Kodak also uses high quality optical encoders. By using a digital, discrete time control system, the electronics cost and size can be reduced, but an optical encoder that produces the high quality sine and cosine signals is still required.
Low cost speed control systems using inexpensive magneto-resistive (MR) encoders are used in video tape recorders. However, these systems are not able to rotate a motor at speeds of less than about 5 RPM. MR signals are inherently analog in nature and, although quasi-sinusoidal, suffer from considerable distortion, amplitude modulation and DC level drift as compared with signals generated by optical encoders. The DC level drift is of particular significance since the DC level uncertainty may range over a few tens of millivolts. However, MR encoders produce signals of a few tens of millivolts peak to peak independent of speed. This means that the peak MR signals may be substantially masked by the uncertainty in the concurrent DC level under conventional operating conditions. Furthermore, current conventional MR encoders are limited to about 1500 sine and cosine wave cycles per revolution in packages of about 1.5 inches (1.5") in diameter.
At rotational frequencies up to the encoder servo loop bandwidth encoder anomalies will generally be manifested as shaft position disturbances. Encoder anomalies at frequencies near the encoder servo natural frequency will, in fact, be harmonically exacerbated.
A particular application that requires precise motor speed control involves the storage and retrieval of data from a linear tape media. Typically a speed controlled motor is used to drive a capstan directly through a speed changing mechanism that is mechanically linked to the tape. Thus, for systems with non-longitudinal data track orientation in a write mode, non-uniform capstan motion causes two undesirable results: (a) track pitch variations; and (b) positional deviations from the desired locus of the head across the tape. In the read mode, track pitch variations reduce the available off-track margin. Track locus variations, to the degree they are not followed, also reduce off-track margins.